1. Field of the Invention
The present invention concerns a storage cell comprising at least one electrode one edge of which is reinforced by an attached strip.
2. Description of the Prior Art
A storage cell comprises an electrode assembly comprising at least one positive electrode and at least one negative electrode disposed face-to-face on respective opposite sides of an electrically insulative separator. The separator is generally a polymer material felt or membrane impregnated with electrolyte.
When the storage cell is being assembled, the electrode of each polarity must be electrically connected to the respective output terminal. The connection is usually made by a thin metal band which may be cut to a required contour and is fixed to the internal part of the terminal and to the conductive support of the electrode.
To make the fixing of the band sufficiently reliable, the edge of the electrode is reinforced, at least in the fixing area, but more reliably so throughout its length. The band can be welded to the reinforcement or integral with it, as described in French patent 2 705 834.
The member reinforcing the electrode is advantageously a thin metal tape applied to one or both faces and parallel to the edge of the electrode. It is welded in place.
At the end of the tape there is a short distance between the edge of the tape and the last weld. This enables one of the straight corners at the end of the tape to bend as a result of mechanical impact during handling, for example by rubbing on the working surface, when the electrodes are being stacked up or the electrode assembly is being assembled. The separator is mechanically weak and easily torn by the resulting sharp corner, and so a short circuit is inevitable. The object of the present invention is to propose a storage cell in which the risk of a short circuit occurring under the conditions just described is minimized.
The present invention consists in a storage cell comprising at least two electrodes with different polarities disposed face-to-face on respective opposite sides of a separator and a metal strip fixed along an upper edge of a first of the electrodes and extending at least as far as one end of the upper edge, wherein the part of a second of the electrodes facing the strip in line with the aforementioned one end is partly cut away by means of a cut-out.
The part of the tape of the second electrode that is cut away is the one whose corner faces the corner of the first electrode which is the origin of the short circuits. For reasons of reliability, the cut-out encroaches partly on the part of the electrode coated with active material.
In a preferred embodiment of the invention, the cut-out has first and second sides merging with each other via a rounded portion and merging via respective rounded portions with an upper edge of the strip and a lateral edge of the second electrode.
The first end of the cut-out is a straight line segment which intersects a lower edge of the strip at an angle greater than 90xc2x0. This avoids the creation of an acute angle at the intersection which is more vulnerable to mechanical impact.
The second side of the cut-out intersects the lateral edge of the second electrode at an angle greater than 90xc2x0, for the same reason as previously.
The cut-out at one end of the strip advantageously has different dimensions to the cut-out at its other end. This slight asymmetry allows for the direction of movement of the tool for cutting the heads of the plates in the metal strip. This facilitates cutting and eliminates the wastage of material along the length of the strip of electrodes.
A single strip 0.2 mm thick, for example, can be applied to one face of the electrode. Two strips, each 0.1 mm thick, for example, can be placed on respective opposite sides of the electrode, and can consist of the same strip bent in two and straddling the edge of the electrode.
The strip is preferably spot welded or knurled wheel welded or ultrasound welded to the first electrode. The weld assures reliable electrical continuity between the current collector of the electrode and the strip.
The electrodes can be of the sintered or non-sintered type. A non-sintered electrode comprises a conductive support and a paste containing the electrochemically active material and a binder.
The electrode support can be a two-dimensional support such as expanded metal, a solid or perforated strip, a grid or a woven fabric, or a three-dimensional support such as a foam or a felt.
The present invention applies to storage cells with an alkaline electrolyte, such as nickel cells, or an organic electrolyte, such as lithium cells. The electrodes can be plane or spiral.